Sandison and his colleagues [see S0-0048] have established that LSD is of great value in the psychotherapy of neuroses. This paper attempts to examine in rather more detail the possible mechanism of action of LSD, in terms of dynamic psychology. An analysis of the material produced under the influence of LSD showed that it bore a striking similarity to the dream and fantasy material of patients undergoing deep analysis. LSD produced an upsurge of unconscious material into consciousness and this material was of great personal significance to the patient. A report is made on two patients for whom little could be done until LSD treatment showed them the contents of the unconscious, and psychotherapy helped them to assimilate these.